FOSSILS OF THE CAMBRIAN. 37 
it is embryonic and representative of one of the earlier stages of growth 
of the species. The second stage, of which an outline figure is given 
on plate xxi, fig. 10, has changed somewhat, but the essential elements re- 
main, and in fig. 11 there is a still greater expansion of the posterior margin 
of the head and the genal angles are carried slightly forward. There are 
probably several intermediate forms between figs. 10 and 11 that have not 
as yet been discovered, but for the present comparison they may be dis- 
pensed with, as in fig. li we have a form that may be compared in the 
contour of the head with our figs. 1 and 3; the genal angles are forward of 
the line of the posterior margin, and strong obtuse angles occur where the 
facial sutures cut the latter at wx. In fig. 12 the normal adult form of the 
head of this species is outlined. Reviewing the development thus hastily 
sketched, it is seen that the form fig. 11 is intermediate between figs. 10 
and 12, and representative of the group of forms found in O. Howelli, be- 
tween fig. 6 and the earlier forms preceding fig. 1 in size, fig. 6 being 
considered as nearest the adult type in the contour of the head. Without 
more data, it is, perhaps, conjectural to consider that the forms preceding 
fig. 1 were like figs. 2 and 4, and that preceding these the earlier stages of 
O. asaphoides were represented, the forms figs. 2, 4, 1, 8, 5 occupying the 
position in development taken by 11 in the latter species. At present no 
other view seems as reasonable, unless it is thought that fig. 2 represents 
the extreme embryonic tendency of the species, a course of development 
the reverse of that of Olenellus asaphoides, or any known species of the tri- 
lobita, and one not at all probable in this instance. 
Il. Relations to other species and genera.—The difficulty met with in 
comparing the abnormal development of this species with that of other 
species in which the thorax and pygidium are known is also felt in study- 
ing its relations to various species and genera in which the structure of the 
entire body is known, restricting as it does all comparisons to the head. 
The species associated with Olenellus Howelli are O. Gilberti, plate ix, 
fig. 16, plate xxi, fig. 13, and O. Iddingsi, plate ix, fig. 12. The smallest 
head of the former, fig. 13, is about the size of fig. 2 of the same plate, but 
does not present the strong embryonic features of the latter. In compari- 
son with the adult head of the same species, plate ix, fig. 16, it is observed 
